1. Field of the Invention
The present invention relates to a zoom lens, and is suitably used for an image pickup apparatus such as a digital camera, a video camera, a TV camera, a monitoring camera, or a silver-salt film camera.
2. Description of the Related Art
As a negative lead type zoom lens in which a lens unit having a negative refractive power is disposed at an object side, a four-unit zoom lens that includes, in order from the object side to an image side, lens units having negative, positive, negative, and positive refractive powers, and that changes a distance between adjacent lens units so as to perform zooming is known (Japanese Patent Laid-Open No. 2006-58584, U.S. Pat. No. 7,777,967, and Japanese Patent Laid-Open No. 2001-343584).
In addition, a five-unit zoom lens that includes, in order from the object side to the image side, first to fifth lens units having negative, positive, positive, negative, and positive refractive powers, and that moves each lens unit so that a distance between adjacent lens units changes in zooming is known (Japanese Patent Laid-Open No. H07-306362).
In these zoom lenses, various focusing methods are used. In Japanese Patent Laid-Open No. 2006-58584, a first lens unit includes, in order from the object side to the image side, a first lens subunit having a negative refractive power and a second lens subunit having a negative refractive power, and the second lens subunit moves in focusing. In U.S. Pat. No. 7,777,967, a fourth lens unit moves in focusing. In Japanese Patent Laid-Open No. 2001-343584, a third lens unit moves in focusing. In Japanese Patent Laid-Open No. H07-306362, a second lens unit moves in focusing.
In the negative lead type zoom lens, it is relatively easy to widen an angle of field, and there are characteristics that a long back focus can be easily obtained. However, a lens configuration is asymmetric with respect to an aperture stop, and therefore it is difficult to correct various aberrations and for example a large amount of aberration variation is generated in focusing, and thus it is difficult to obtain a high optical performance.
Particularly, in order to perform high speed focusing, the aberration variation in focusing tends to be increased in an inner focus method of performing the focusing by a small and lightweight lens unit disposed at the image side relative to the first lens unit.
In order to widen the angle of field, reduce a whole of a lens system, and perform the high speed focusing by using a small and lightweight lens unit, it is important to appropriately set a refractive power (a power) of each lens unit, a lens configuration, or the like.